Angles on walls in homes are rarely as precise as the angle on your miter box. Have you ever used a coping saw. This couple of minutes might very well be worth your time. https://www.youtube....h?v=W8ccy0wLErQ This allows for all of those inside corners that invariably wont match your miter saw.
That works for the inside corner, but he still has to get the outside corner made too. It's always a crap shoot on an angled wall! I usually try to find a couple small scraps and play with the angles until it fits decently! Then it's a matter of getting the length right!